This invention relates generally to transparent conducting oxides, and more particularly, to such compositions and related structures having p-type conductivities and methods for their preparation under hydrothermal reaction conditions.
Transparent conducting oxides (TCOs) are degenerate wide band-gap semiconductors with conductivities comparable to metals, but are transparent over the visible and IR regions. Currently, the best known and industrially useful TCOs are doped ZnO, SnO2 and In2O3, all of which are n-type semiconductors. For example, in thin film forms, Sn-doped indium oxide has n-type conductivity on the order of 103 S/cm and an average transmittance higher than 85% in the visible light range. By comparison, in thin film form, the p-type conductivity of CuAlO2 is about 1 S/cm and about 10−3 S/cm in bulk form. (H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi and H. Hosono, Nature, 389, 939-942 (1997). p-Type Electrical Conduction in Transparent Thin Films of CuAlO2. F. A. Benko and F. P. Koffyberg, J. Phys. Chem. Solids, 45, 1, 57-59, (1984). Opto-electronic properties of CuAlO2.)
Many ternary oxides with an AIBIIIO2 composition adopt the delafossite (CuFeO2) structure, where A is either Cu, Pd, Pt or Ag and B is a trivalent metal with 0.53<r(BVI3.)<1.09 Å. These delafossite-type oxides comprise a rich family of compounds with interesting luminescence properties and applications in areas of catalysis or electrocatalysis. Until recently, only SrCu2O2 and nitrogen-doped ZnO were the only known p-type TCOs. The recent discovery of simultaneous p-type conductivity and transparency in CuAlO2 has heightened interest in CuMO2 compounds and in particular those having delafossite structures. Owing to this unique dual property, TCOs find various technological applications in solar cells, optoelectronic materials, energy-efficient windows, gas sensors and flat panel displays, among others. The discovery of new p-type TCOs will open up new application possibilities that are simply not feasible with unipolar n-type materials alone.
To date, most of the bulk CuAlO2 syntheses correspond to direct- or cation exchange reactions in the solid phase. [B. U. Köhler and M. Jansen, Z. Anorg. Allg. Chem., 543, 73-80 (1986). Darstellung and Strukturdaten von Delafossiten CuMO2 (M=Al, Ga, Sc, Y); T. Ishiguro, A. Kitazawa, N. Mizutani and M. Kato, J. Solid State Chem., 40, 170-174 (1981). Single-crystal growth and crystal structure refinement of CuAl02.>>; H. Hahn and C. Lorent, Z. Anorg. Allg. Chem., 279, 281 (1955); B. Köhler and M. Jansen, Z. Krist., 129, 259 (1983); J. P. Doumerc, A. Amar, A. Wichainchai, M. Pouchard and P. Hagenmuller, J. Phys. Chem. Solids, 48, 1, 37-43 (1987). Sur Quelques Nouveaux Composés de Structure de type Delafossite.] Based on the work of Croft et al. on AgFeO2 [W. J. Croft, N. C. Tombs and R. E. England, Acta Chryst., 17, 313 (1964). Crystallographic data for pure Crystalline Silver Ferrite.], Shannon et al. [R. D. Shannon, D. B. Rogers and C. T. Prewitt., Inorg. Chem., 10, 4, 713-727 (1971). Chemistry of Noble Metal Oxides. I. Syntheses and properties of ABO2 Delafossite compounds. II. Crystal structures of PtCoO2, PdCoO2, CuFeO2 and AgFeO2. III. Electrical transport properties and crystal chemistry of ABO2 compounds with the delafossite structure.] reported the first hydrothermal synthesis of CuAlO2 as well as other ABO2 compounds, using a thin-walled platinum tube at 500° C. with 3000 atm of externally applied pressure. However, in addition to the reaction conditions (high temperature and pressure, with prolonged reaction times), a limitation of this technique is that many of the delafossite-type compounds could not be isolated as single phases, without a separate isolation or leaching procedure.